Low-noise spectroscopic imaging system using substantially coherent illumination

ABSTRACT

A spectral imaging device ( 12 ) includes an image sensor ( 28 ), a tunable light source ( 14 ), an optical assembly ( 17 ), and a control system ( 30 ). The optical assembly ( 17 ) includes a first refractive element ( 24 A) and a second refractive element ( 24 B) that are spaced apart from one another by a first separation distance. The refractive elements ( 24 A) ( 24 B) have an element optical thickness and a Fourier space component of the optical frequency dependent transmittance function. Further, the element optical thickness of each refractive element ( 24 A) ( 24 B) and the first separation distance are set such that the Fourier space components of the optical frequency dependent transmittance function of each refractive element ( 24 A) ( 24 B) fall outside a Fourier space measurement passband.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/109,570, filed on Jul. 1, 2016, entitled “LOW-NOISE SPECTROSCOPICIMAGING SYSTEM USING SUBSTANTIALLY COHERENT ILLUMINATION”. U.S.application Ser. No. 15/109,570 is a 371 of and claims priority from PCTApplication Serial No. PCT/US2015/011884, filed Jan. 18, 2015, entitled“LOW-NOISE SPECTROSCOPIC IMAGING SYSTEM USING SUBSTANTIALLY COHERENTILLUMINATION”. PCT Application Serial No. PCT/US2015/011884 claimspriority on U.S. Provisional Application Ser. No. 61/929,050, filed Jan.18, 2014 and entitled “A LOW-NOISE SPECTROSCOPIC IMAGING SYSTEM USINGCOHERENT ILLUMINATION”. As far as permitted, the contents of U.S.application Ser. No. 15/109,570, PCT Application Serial No.PCT/US2015/011884, and U.S. Provisional Application Ser. No. 61/929,050are incorporated herein by reference. Further, as far as permitted, thecontents of PCT Application Serial No. PCT/US2012/061987 is incorporatedherein by reference.

BACKGROUND

Microscopes are often used to analyze a sample in order to evaluatecertain details and/or properties of the sample that would not otherwisebe visible to the naked eye. Additional information on the chemicalproperties of the sample can be obtained by illuminating and observingthe sample with discrete optical frequencies of monochromatic laserradiation. Samples that can be analyzed this way include human tissueand cells, explosive residues, powders, liquids, solids, polymers, inks,and other materials. A human tissue sample may be analyzed for thepresence of cancerous cells and/or other health related conditions.Other materials may be analyzed for the presence of explosive residuesand/or other dangerous substances.

Unfortunately, spectral images generated from the samples with existingspectral microscopes can sometimes be of insufficient quality to enablefull and effective analysis of the samples. Thus, it is desired toimprove the resolution and quality of the spectral images of the samplesthat are being generated.

SUMMARY

The present invention is directed toward a spectral imaging device forgenerating a spectral cube of a sample, the spectral imaging devicehaving a desired spectral resolution. The spectral imaging deviceincludes an image sensor, a tunable optical source, an optical assembly,and a control system. The image sensor includes a two-dimensional arrayof sensors that are used to construct a two-dimensional image. Thetunable optical source generates an illumination beam that follows abeam path from the tunable optical source to the sample and from thesample to the image sensor, the illumination beam having a spectralwidth that is equal to or less than the desired spectral resolution. Theoptical assembly includes a plurality of refractive elements that arepositioned along the beam path between the tunable optical source andthe image sensor, the optical assembly including a first refractiveelement and a second refractive element that are spaced apart from oneanother by a first separation distance. The control system controls thetunable optical source to generate a plurality of discrete opticalfrequencies within a desired tuning range, and controls the image sensorto construct a plurality of two-dimensional images.

In one embodiment, the first refractive element has a first elementoptical thickness and a first Fourier space component of the opticalfrequency dependent transmittance function, and the second refractiveelement has a second element optical thickness and a second Fourierspace component of the optical frequency dependent transmittancefunction. Further, the element optical thickness of each refractiveelement and the first separation distance are set such that the Fourierspace components of the optical frequency dependent transmittancefunction of each refractive element fall outside a measurement passband.As provided herein, the measurement passband is equal to the reciprocalof two times the desired spectral resolution.

In certain embodiments, the element optical thickness, t, for the firstand second refractive elements is defined by either t≥1/(2nΔv) ort≤1/(2n(v₂−v₁)); and wherein the first separation distance, d, isdefined by either d≥1/(2nΔv) or d≤1/(2n(v₂−v₁)); where n is refractiveindex of the respective refractive element, Δv is the desired spectralresolution, v₁ is a lower bound of the desired tuning range, and v₂ isan upper bound of the desired tuning range.

In one embodiment, the control system collects a spectral image with aspectral resolution and optical frequency step size that is less than orequal to the free spectral range of the refractive element having theshortest free spectral range in the beam path divided by two; and alow-pass filter is subsequently applied to the spectral response of eachpixel in the spectral image to create an output spectral image at alower spectral resolution. The control system can apply at least one ofa running average and a Gaussian filter. Further, the data can bedecimated.

Stated in another fashion, the control system can control the tunablelight source to generate a set of discrete optical frequencies near atarget optical frequency, with adjacent optical frequencies of the setbeing spaced apart an optical frequency step, the optical frequency stepbeing sufficiently small such that it does not produce aliasing of theFourier components of the optical frequency dependent transmittancefunction of the parasitic etalons contained along the beam path into themeasurement passband; and the control system controls the image sensorto construct a separate, two dimensional image at each discrete opticalfrequency, and the control system constructs an output image of thesample for the target optical frequency using the separate twodimensional images at each discrete optical frequency.

In another embodiment, the control system modulates the tunable lightsource to generate a set of discrete optical frequencies near a targetoptical frequency to produce a maximum optical frequency modulation,Δv_(modulation), about the target optical frequency which satisfies thefollowing prescription: Δv_(modulation)=±ηΔv/2, where η is a constanthaving a value of greater than or equal to 0.1 and less than or equal to100, and Δv is the desired optical frequency spectral resolution. Statedin another fashion, the control system modulates the tunable lightsource to generate a set of discrete optical frequencies about andthrough a target optical frequency at an optical frequency modulationrate, and wherein the image sensor captures the output image during acapture time that is longer than the inverse of the optical frequencymodulation rate.

In certain embodiments, tunable optical source emits a temporallycoherent illumination beam and the desired tuning range is themid-infrared range.

In another embodiment, the present invention is directed to a spectralimaging device for generating a spectral cube of a sample, the spectralimaging device having a desired spectral resolution, the spectralimaging device comprising: an image sensor component that includes atwo-dimensional array of sensors that are used to construct atwo-dimensional image; a tunable optical source component that generatesan illumination beam that follows a beam path from the tunable opticalsource to the sample and from the sample to the image sensor, theillumination beam having a spectral width that is equal to or less thanthe desired spectral resolution; an optical assembly including aplurality of refractive element components that are positioned along thebeam path between the tunable optical source component and the imagesensor component, each refractive element component having an elementoptical thickness; and a control system that controls the tunableoptical source to generate a plurality of discrete optical frequencieswithin a desired tuning range, and controls the image sensor toconstruct a plurality of two-dimensional images; wherein the componentsare spaced apart from one another along the beam path; wherein aseparate, separation distance separates adjacent components along thebeam path; wherein each element optical thickness is designed to have aFourier space component of the optical frequency dependent transmittancefunction that falls outside a measurement passband; wherein eachseparation distance is designed to have a Fourier space component of theoptical frequency dependent transmittance function that falls outsidethe measurement passband; and wherein the measurement passband is equalto the reciprocal of two times the desired optical frequency spectralresolution.

In still another embodiment, the present invention is directed to aspectral imaging device for generating a spectral cube of a sample, thespectral imaging device having a desired spectral resolution, thespectral imaging device comprising: an image sensor that includes atwo-dimensional array of sensors that are used to construct atwo-dimensional image; a tunable light source that generates anillumination beam that follows a beam path from the tunable opticalsource to the sample and from the sample to the image sensor, theillumination beam having a spectral width that is equal to or less thanthe desired spectral resolution; an optical assembly including aplurality of refractive elements that are positioned along the beam pathbetween the tunable optical source and the image sensor; and a controlsystem that controls the tunable optical source to generate a pluralityof discrete optical frequencies within a desired tuning range, andcontrols the image sensor to construct a plurality of two-dimensionalimages; wherein the control system collects a spectral image with aspectral resolution and an optical frequency step size that is less thanor equal to the free spectral range of the refractive element having theshortest free spectral range in the beam path divided by two; andwherein a low-pass filter is subsequently applied to the spectralresponse of each pixel in the spectral image to create an outputspectral image at a lower spectral resolution.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of this invention, as well as the invention itself,both as to its structure and its operation, will be best understood fromthe accompanying drawings, taken in conjunction with the accompanyingdescription, in which similar reference characters refer to similarparts, and in which:

FIG. 1A is a simplified schematic illustration of a sample and anembodiment of a spectral imaging microscope having features of thepresent invention;

FIG. 1B is a simplified schematic illustration of the spectral imagingmicroscope of FIG. 1A, in a transmission mode;

FIG. 1C is a simplified schematic illustration of the spectral imagingmicroscope of FIG. 1A, in a reflection mode;

FIG. 2 is a simplified schematic illustration of a spectral imagingdevice;

FIG. 3 is a simplified graph that illustrates transmittance versuswavenumber for a refractive element having two surfaces with varyingdegree of surface reflectances;

FIG. 4A is a simplified illustration of an ideal image a sample;

FIG. 4B is a simplified illustration of a first non-ideal image of thesample;

FIG. 4C is a simplified illustration of a second non-ideal image of thesample;

FIG. 5A is a graph of transmittance versus wavenumber in the opticalfrequency space for two different thickness of refractive elements;

FIG. 5B is a graph that illustrates the power spectral density versusFourier space frequency in the Fourier Space of the two refractiveelements shown in FIG. 5A;

FIG. 6A is a simplified illustration of the optical frequency space foran spectral imaging device;

FIG. 6B is a simplified illustration of the corresponding Fourier spacefor the spectral imaging device;

FIG. 7A is a graph that illustrates optical frequency versus time;

FIG. 7B is another graph that illustrates optical frequency versus time;

FIG. 7C illustrates a plurality of preliminary images used to generate aseparate output image for each target optical frequency;

FIG. 8A is a graph of transmittance versus wavenumber in the opticalfrequency space for two different refractive elements having differentthickness with solid circles and open squares showing sampled datapoints;

FIG. 8B is a graph in the Fourier frequency space of the sampledtransmittance versus wavenumber plots shown in FIG. 8A;

FIG. 9 is a graph in the optical frequency space of a sampled raw signaldata and a sampled, averaged, and decimated signal data;

FIG. 10A is a graph that illustrates optical frequency versus time;

FIG. 10B is a another graph that illustrates optical frequency versustime;

FIG. 10C illustrates an output image;

FIG. 11 includes an upper graph with an illustration of a narrow opticalfrequency distribution, a middle graph with an illustration of a verybroad optical frequency distribution of an output beam, and a lowergraph having a schematic illustration of a plurality of narrow opticalfrequency pulses of energy generated in a relatively short period oftime and the dashed line showing the time-averaged optical frequency ofthe series of pulses;

FIG. 12A is an image captured without noise reduction methods providedherein; and

FIG. 12B is a captured image using the spectral image device providedherein.

DESCRIPTION

FIG. 1A is a simplified schematic illustration of a sample 10 and anembodiment of a spectral imaging device 12, e.g., a spectral imagingmicroscope, having features of the present invention. In particular, thespectral imaging device 12 can be used to quickly and accurately acquirea spectral cube 13 (illustrated as a box) of the sample 10 that can beused to analyze and evaluate the various properties of the sample 10. Asprovided herein, in certain embodiments, the spectral imaging device 12is uniquely designed to generate a plurality of high resolution, twodimensional, output images 13A, 13B, 13C (only three are illustrated asboxes) of the sample 10 that are used to create the spectral cube 13 forthe sample 10. The term “image” as used herein shall mean and include atwo-dimensional photograph or screen display, or a two-dimensional arrayof data that can be used to generate the two-dimensional photograph orscreen display.

As an overview, as discussed in greater detail herein below, thespectral imaging device 12 includes a Fourier space measurement passband (defined by the reciprocal of the spectral resolution and referredto herein simply as a “pass band”) and the spectral imaging device 12can include certain structural features that cause certain noise sourcesto fall outside the measurement pass band. With this design, thespectral imaging device 12 can effectively inhibit noise sources fromadversely impacting the spectral resolution and image quality of eachoutput image 13A, 13B, 13C. Additionally and/or alternatively, thespectral imaging device 12 can utilize algorithms and/or methodologiesthat further inhibit noise sources from adversely impacting theresolution and image quality of each output image 13A, 13B, 13C.

As provided herein, the sample 10 can be analyzed and evaluated in astatic sense, i.e. where the properties of the sample 10 aresubstantially unchanged over the measurement period, and/or in a dynamicsense, i.e. where the properties of the sample 10 are evolving over themeasurement period. In the static case, a one-dimensional spectra isproduced for every pixel position of the two-dimensional output image13A, 13B, 13C to yield a three-dimensional spectral cube 13. In thedynamic case, a fourth dimension of time is added to yield afour-dimensional spectral matrix 13.

The fidelity of the data of the spectral cube 13 can be characterized bythe repeatability of the spectral data at each pixel location, overmultiple trials. Each trial has a unique data collection start time.Because the source intensity may vary strongly across the sample 10 aswell as across the optical frequency band of interest, one or morefeatureless background spectral cubes (without the sample) may begenerated and used to normalize the signal spectral cube by taking theratio of the signal spectral cube to the background spectral cube. Ifthe frequencies are collected in an ordered array, then the ratio isreferred to as the image transmittance.

As provided herein, a ratio of two background spectral cubes takenwithout the sample 10, at different times, can be used to produce asystem transmittance spectral cube (not shown). Comparing thepixel-by-pixel transmittance over many trials and over opticalfrequencies is a suitable means for characterizing the intrinsicsignal-to-noise ratio (SNR) of the spectral imaging device 12. Anon-exclusive example of an acceptable measure of the intrinsic systemSNR is the reciprocal of the variance of the transmittance over aspecified spectral range for two randomly selected spectral cubecollection trials taken at different times.

The sample 10 can be a variety of things, including mammalian blood,mammalian blood serum, mammalian cells, mammalian tissue, mammalianbiofluids, and their animal counterparts, plant matter, bacteria,polymers, hair, fibers, explosive residues, powders, liquids, solids,inks, and other materials commonly analyzed using spectroscopy andmicroscopy. More particularly, in certain non-exclusive applications,the sample 10 can be human blood serum, and the spectral imagingmicroscope 12 can be utilized for rapid screening of the serum sample 10for the presence of disease and/or other health related conditions;and/or the spectral imaging microscope 12 can be utilized in certainforensic applications such as rapid screening of the sample 10 for thepresence of explosive residues and/or other dangerous substances.Additionally, when positioned substantially within the spectral imagingmicroscope 12 for purposes of analysis, the sample 10 can be present byitself, or the sample 10 can be held in place using one or more slides(not shown), e.g., infrared transparent slides.

Further, the sample 10 can be thin enough to allow study throughtransmission of an illumination beam, e.g., an infrared illuminationbeam, through the sample 10 (i.e. in transmission mode), or the sample10 can be an optically opaque sample that is analyzed through reflectionof an illumination beam, e.g., an infrared illumination beam, by thesample 10 (i.e. in reflection mode). Still further, the sample 10 can bethin enough to allow study through transflection of an illuminationbeam, e.g., an infrared illumination beam can pass through the sample,reflect on the surface of a reflective substrate, and again pass throughthe sample 10, the illumination beam being double attenuated. Forexample, in the embodiment illustrated in FIG. 1A, the spectral imagingmicroscope 12 can be utilized in transmission mode and/or reflectionmode, and data can be acquired using a transmission, reflection, ortransflection methodology.

It should be appreciated that the spectral imaging device 12 can beutilized in a variety of potential applications. For example, suchapplications can include, but are not limited to, spectralhistopathology and cytopathology, hematology, pharmaceutical drugdevelopment and process control, detection of biochemical warfare agentsand other hazardous materials, materials science, and polymer sciencedevelopment.

The design of components of the spectral imaging device 12 can be variedto achieve the desired characteristics of the spectral imaging device12. In one embodiment, the spectral imaging device 12 is an infraredspectral imaging microscope that uses tunable laser radiation tointerrogate the sample 10.

In the non-exclusive embodiment illustrated in FIG. 1A, the spectralimaging microscope 12 includes (i) a tunable optical source 14 thatgenerates and/or emits an optical illumination beam 16, (ii) an opticalassembly 17 that includes an illumination optical assembly 18 and animaging optical assembly 24, (iii) a beam steerer assembly 20 thatsteers the illumination beam 16 along a desired beam path, (iv) anillumination switch 22 that is controlled by a user (not shown) so thatthe illumination beam 16 can be alternatively directed at the sample 10in a transmission mode or a reflection mode, (v) a beam splitter 26,(vi) an image sensor 28 that captures information to create the outputimages 13A, 13B, 13C and the spectral cube 13 of the sample 10; and(vii) a control system 30 that is electrically connected to and controlsmany of the components of the spectral imaging device 12. One or more ofthese devices can be referred to as a component.

It should be noted that the spectral imaging microscope 12 can bedesigned with more or fewer components than are illustrated in FIG. 1A,and/or the components can be organized in another fashion thanillustrated in FIG. 1A. For example, the spectral imaging microscope 12can include a multiple position lens turret (not shown) that includesone or more mid-infrared objective lens assemblies with differentcharacteristics, and/or one or more objective lens assemblies that workoutside the mid-infrared spectral range. Additionally, for example, theoptical assembly 17 can be designed without the illumination opticalassembly 18.

Moreover, the spectral imaging device 12 can include an image display 31(illustrated as a box), e.g. an LED display, that displays one or moreof the output images 13A, 13B, 13C in real time, and/or subsequentlydisplays the spectral cube 13.

In certain embodiments, the spectral imaging microscope 12 has arelatively high resolution, high numerical aperture (“NA”), and arelatively large field of view (“FOV”). This allows for the collectionof data from relatively large samples. This will improve the speed inwhich the sample is analyzed. As one non-exclusive example, the spectralimaging microscope 12 can have NA of 0.7, a magnification of 12.5×, anda FOV of approximately 650 μm×650 μm, with a sample-referred pixel sizeof 1.36 μm.

In certain embodiments, the tunable optical source 14 includes a lasersource that emits a substantially temporally coherent illumination beam16 (e.g. a laser beam) that is usable for illuminating and analyzing thesample 10 in transmission mode and/or in reflection mode. Theillumination beam 16 is made up of a plurality of illumination rays 16Athat follow a beam path from the optical source 14 to the sample 10 andfrom the sample 10 to the image sensor 28. Further, the illuminationrays 16A can have a single, discrete center optical frequency that iswithin a desired tuning range of the optical source 14. Alternatively,the optical source 14 can be controlled by the control system 30 to varythe discrete center optical frequency of the illumination rays 16A overtime within the desired tuning range.

In certain embodiments, the optical illumination beam 16 has a spectralwidth that is equal to or less than a desired spectral resolution(represented by the delta v “Δv”) of the spectral imaging device 12. Thebuilder of the spectral imaging device 12 selects the desired spectralresolution and builds the system accordingly. For example, the desiredspectral resolution of the spectral imaging device 12 can be four cm⁻¹wavenumbers (Δv=4 cm⁻¹). Alternatively, for example, the desiredspectral resolution can be 2, 3, 4, 4.1, 5, 5.25, 6, 7, 8, 9, 10, or 16cm⁻¹ wavenumbers. However, other desired spectral resolutions can beutilized.

In certain non-exclusive embodiments, the tunable optical source 14 is atunable mid-infrared optical source that directly generates and emitsthe illumination beam 16 having a center optical frequency that is inthe mid-infrared (“MIR”) range. In this example, the desired tuningrange is the MIR range. Further, as used herein, term “MIR range” shallmean and include the spectral region or spectral band of betweenapproximately two and twenty micrometers (2-20 μm) in wavelength or fivethousand to 500 wavenumbers (5000-500 cm⁻¹). The mid-infrared range isparticularly useful to spectroscopically interrogate the sample 10 sincemany samples 10 are comprised of molecules or groups of molecules thathave fundamental vibrational modes in the MIR range, and thus presentstrong, unique absorption signatures within the MIR range.Alternatively, the tunable optical source 14 can be designed to generatethe illumination beam 16 having a center optical frequency of greaterthan twenty or less than two micrometers.

Moreover, in alternative embodiments, the tunable optical source 14 canbe either a pulsed laser or a continuous wave (CW) laser. For a pulsedoptical source 14, the illumination beam 16 will include a plurality ofpulses of illumination rays 16A that follow the beam path from thetunable optical source 14 to the sample 10 and from the sample 10 to theimage sensor 28. Further, the pulses of illumination rays 16A can have adiscrete center optical frequency that is within the MIR range.

In certain embodiments, the discrete center optical frequency of theoptical illumination source 16A can vary over time over the entire or aportion of the MIR range to analyze the sample 10 over the desiredspectral range. For example, for a pulsed optical source 14, the opticalsource 14 can be tuned to generate an optical illumination beam 16 thatconsists of a set of sequential, specific output pulses of light havingdifferent, discrete center optical frequency that span the entire orjust a portion of the MIR range. For example, the optical source 14 canbe tuned to a first position and one or more pulses can be generatedhaving approximately the same first center optical frequency (“firsttarget optical frequency”). Subsequently, the optical source 14 can betuned to a second position and one or more pulses can be generatedhaving approximately the same second center optical frequency (“secondtarget optical frequency”) that is different from the first centeroptical frequency. Next, the optical source 14 can be tuned to a thirdposition and one or more pulses can be generated having approximatelythe same third center optical frequency (“third target opticalfrequency”) that is different from the first and second center opticalfrequency. This process can be repeated to a plurality of additionaltarget optical frequencies throughout a portion or the entire MIR range.As non-exclusive examples, the number of pulses at each discrete opticalfrequency can be 1, 5, 10, 50, 100, 200, 500, 1000, 10,000 or more.Alternatively, the tunable optical source 14 can be operated in acontinuous wave fashion at each target optical frequency.

The number of discrete target optical frequencies in the set used toacquire the spectral cube 13 can also vary according to the sample 10.As non-exclusive examples, the number of discrete target opticalfrequencies in the mid-infrared range utilized to acquire the spectralcube 13 can be approximately 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25,40, 200, 226, 400, 552 or 4000. As provided herein, the term “targetoptical frequency step” shall mean the smallest allowed differencebetween adjacent target optical frequencies. In alternative,non-exclusive embodiments, the target optical frequency step can beapproximately 0.1, 0.2, 0.25, 0.33, 0.5, 0.67, 0.7, 1.0, 2.0, 4.0, 8.0,or 16, wavenumbers.

In certain, non-exclusive embodiments, the illumination beam 16 from theMIR optical source 14 has an optical spectral full width at half maximum(FWHM) of less than approximately 0.01, 0.05, 0.1, 0.25, 0.5, 1.0, 2.0,or 4 cm⁻¹.

In certain embodiments, the control system 30 can control the opticalsource 14 to be tuned so that the illumination beam 16 has the firsttarget optical frequency, and the control system 30 can control theimage sensor 28 to capture the first image 13A with the sample 10illuminated at the first target optical frequency. Subsequently, thecontrol system 30 can control the optical source 14 to be tuned so thatthe illumination beam 16 has the second target optical frequency and thecontrol system 30 can control the image sensor 28 to capture the secondimage 13B with the sample 10 illuminated at the second target opticalfrequency. This process is repeated for each target optical frequencyuntil a plurality of images 13A, 13B, 13C, are collected across theoptical frequency range of interest, thus generating a spectral cube 13.

Additionally, the optical source 14 of FIG. 1A can include multipleindividual lasers that span a portion or all of the desired mid-infraredspectral range. A description of a optical source 14 that includesmultiple individual lasers is described in U.S. patent application Ser.No. 13/949,159, entitled “Laser Source With A Large Spectral Range”filed on Jul. 23, 2013. As far as permitted, the contents of U.S. patentapplication Ser. No. 13/949,159 are incorporated herein by reference.The optical source 14 can utilize a variety of methods to rapidly switchbetween the target optical frequencies. These include techniques such asrapid tuning mechanisms, galvo-controllled mirrors to switch betweendifferent laser modules, or spectral beam combining techniques ofmultiple laser modules and subsequent time-division multiplexing oflaser illumination.

In one, non-exclusive embodiment, the optical source 14 is an externalcavity laser that includes a rigid laser frame 32, a gain medium 34, acavity optical assembly 36, an output optical assembly 38, and awavelength selective (“WS”) feedback assembly 40 (e.g., a movablegrating).

The design of the gain medium 34 can be varied pursuant to the teachingsprovided herein. In one, non-exclusive embodiment, the gain medium 34directly emits the illumination beam 16 without any frequencyconversion. As a non-exclusive example, the gain medium 34 can be asemiconductor type laser. As used herein, the term semiconductor shallinclude any solid crystalline substance having electrical conductivitygreater than insulators but less than good conductors. Morespecifically, in certain embodiments, the gain medium 34 is a QuantumCascade (QC) gain medium, an Interband Cascade (IC) gain medium, or amid-infrared diode. Alternatively, another type of gain medium 34 can beutilized.

In FIG. 1A, the gain medium 34 includes (i) a first facet that faces thecavity optical assembly 36 and the WS feedback assembly 40, and (ii) asecond facet that faces the output optical assembly 38. In thisembodiment, the gain medium 34 emits from both facets. In oneembodiment, the first facet is coated with an anti-reflection (“AR”)coating and the second facet is coated with a reflective coating. The ARcoating allows light directed from the gain medium 34 at the first facetto easily exit the gain medium 34 as an illumination beam directed atthe WS feedback assembly 40; and allows the illumination beam reflectedfrom the WS feedback assembly 40 to easily enter the gain medium 34.

The illumination beam 16 exits from the second facet. The reflectivecoating on the second facet reflects at least some of the light that isdirected at the second facet from the gain medium 34 back into the gainmedium 34. In one non-exclusive embodiment, the AR coating can have areflectivity of less than approximately 2 percent, and the reflectivecoating can have a reflectivity of between approximately 2-95 percent.In this embodiment, the reflective coating acts as an output coupler(e.g., a first end) for the external cavity.

The cavity optical assembly 36 is positioned between the gain medium 34and the WS feedback assembly 40 along a lasing axis, and collimates andfocuses the light that passes between these components. For example, thecavity optical assembly 36 can include a single lens or more than onelens. For example, the lens can be an aspherical lens having an opticalaxis that is aligned with the lasing axis. In one embodiment, to achievethe desired small size and portability, the lens has a relatively smalldiameter. The lens can comprise materials selected from the group of Ge,ZnSe, ZnS, Si, CaF2, BaF2 or chalcogenide glass. However, othermaterials may also be utilized.

The output optical assembly 38 is positioned along the lasing axis. Inthis design, the output optical assembly 38 collimates and focuses theillumination beam 16 that exits the second facet of the gain medium 34.For example, the output optical assembly 38 can include a single lens ormore than one lens that are somewhat similar in design to the lens ofthe cavity optical assembly 36.

The WS feedback assembly 40 reflects the light back to the gain medium34, and is used to precisely select and adjust the lasing frequency(wavelength) of the external cavity and the center optical frequency ofthe illumination beam 16. Stated in another fashion, the WS feedbackassembly 40 is used to feed back to the gain medium 34 a relativelynarrow band optical frequency which is then amplified in the gain medium34. In this manner, the illumination beam 16 may be tuned with the WSfeedback assembly 40 without adjusting the gain medium 34. Thus, withthe external cavity arrangements disclosed herein, the WS feedbackassembly 40 dictates what optical frequency will experience the mostgain and thus dominate the optical frequency of the illumination beam16.

A number of alternative embodiments of the WS feedback assembly 40 canbe utilized. In FIG. 1A, the WS feedback assembly 40 is spaced apartfrom the gain medium 34 and defines a second end of the external cavity.In this embodiment, the external cavity extends from the output coupler(reflective coating) on the second facet to the WS feedback assembly 40.The term external cavity is utilized to designate the WS feedbackassembly 40 is positioned outside of the gain medium 34.

In some embodiments, the WS feedback assembly 40 includes a diffractiongrating 40A and a grating mover 40B that selectively moves (e.g.,rotates) the diffraction grating 40A to adjust the lasing frequency ofthe gain medium 34 and the optical frequency of the illumination beam16. The diffraction grating 40A can be continuously monitored with anencoder 40C that provides for closed loop control of the grating mover40B. With this design, the optical frequency of the illumination beam 16can be selectively adjusted in a closed loop fashion so that the sample10 can be imaged at many different, precise, selectively adjustableoptical frequencies throughout a portion or the entire MIR spectrum.

The control system 30 controls the operation of the tunable opticalsource 14 including the electrical power to the grating mover 40B, andthe electrical power that is directed to the gain medium 34 (e.g.,controls the gain medium 34 by controlling the electron injectioncurrent). Further, the control system 30 can control the image sensor 28to control the timing of the capture of the images 13A, 13B, 13C. Forexample, the control system 30 can include one or more processors and/orstorage devices.

The collection of an accurate spectral cube 13 requires that the opticalfrequency of the optical illumination beam be precisely known as thelaser is tuned. In certain embodiments, the control system 30 directsthe pulses of power to the gain medium 34 based on the position signalreceived from the encoder 40C. Stated in another fashion, the controlsystem 30 can direct one or more pulses of power to the gain medium 34at each of the plurality of alternative device positions so that thelaser generates the set of discrete target optical frequencies. In thisembodiment, the control system 30 can direct one or more pulses of powerto the gain medium 34 upon receipt of each new position signal. As aresult thereof, the specific optical frequency of the pulses will not beinfluenced by variations in speed of the grating mover 40B.

The duration of each pulse of power directed by the control system 30 tothe gain medium 34 can also be varied. In alternative, non-exclusiveembodiments, the control system 30 can control each pulse of power tohave a duration of approximately 10, 25, 50, 75, 100, 150, 200, 300,400, 500, 600 or 700 nanoseconds.

Once the tunable optical source 14 has emitted the illumination beam 16,the illumination beam 16 is directed toward the sample 10 so that thesample 10 may be properly and effectively illuminated by theillumination beam 16. For example, when the spectral imaging microscope12 is operating in transmission mode, the illumination beam 16 isdirected toward the sample 10 in order to properly and effectivelyilluminate the sample 10. In this example, the illumination rays 16Athat are transmitted through the sample 10 are referred to astransmitted rays 16T (also illustrated more clearly in FIG. 1B). As willbe discussed in further detail herein below, FIG. 1B is a simplifiedschematic illustration of a transmission beam path of the illuminationbeam 16 from the optical source 14 of the spectral imaging microscope 12of FIG. 1A to the image sensor 28, with the sample 10 being interrogatedvia transmission of the illumination beam 16 through the sample 10.

In another example, when the spectral imaging microscope 12 is operatingin reflection mode, the illumination beam 16 is directed toward thesample 10 in order to properly and effectively illuminate the sample 10.In this example, the illumination rays 16A that are reflected off of thesample 10 are referred to as reflected rays 16R (also illustrated moreclearly in FIG. 1C). As will be discussed in further detail hereinbelow, FIG. 1C is a simplified schematic illustration of an alternativereflection beam path of the illumination beam 16 from the tunableoptical source 14 of the spectral imaging microscope 12 of FIG. 1A tothe image sensor 28, with the sample 10 being interrogated viareflection of the illumination beam 16 off of the sample 10.

In the embodiment illustrated in FIG. 1A, when operating in transmissionmode, the illumination beam 16 exiting the tunable optical source 14 isdirected with a portion of the illumination optical assembly 18, i.e. atransmission illumination optical assembly 18T (illustrated more clearlyin FIG. 1B), toward the sample 10 so as to impinge on the sample 10. Inone embodiment, the transmission illumination optical assembly 18T caninclude one or more optical, refractive elements, e.g., lenses and/orwindows (three such refractive elements are illustrated in FIG. 1A),that direct the illumination beam 16 at the sample 10. Further, incertain embodiments, the refractive elements are operable in the MIRrange. Moreover, as described in greater detail herein below, pursuantto the teachings of the present invention, the refractive elements canhave thicknesses and spacing (i.e. separation) that inhibitwavelength-dependent noise, e.g., parasitic etalon modulations, fromadversely impacting the image quality and optical spectral resolution ofthe spectra generated from the set of wavelength dependent spectralimages 13A, 13B, 13C of the sample 10 that are being generated.

It should be appreciated that the fluid, e.g., air or another suitablefluid, that fills the spacing between the optical elements of thetransmission optical assembly 18T also functions as optical elementsthat can be refractive in the MIR range.

In certain embodiments, the transmission illumination optical assembly18T can be used to transform, i.e. to increase (magnify) or decrease,the size and profile of the illumination beam 16 to match andsimultaneously illuminate a desired transmission illuminated area on thesample 10. Stated another way, the transmission illumination opticalassembly 18T can be used to condition and focus the illumination beam 16so that the illumination beam 16 has the correct or desired size andbeam profile on the sample 10. In certain embodiments, the size of thetransmission illuminated area of the sample 10 is tailored to correspondto the design of the image sensor 28 and the imaging optical assembly24. As non-exclusive examples, the desired transmission illuminatedcircular area bounded by a diameter that is approximately 50, 100, 200,250, 500, 600, 650, 700, 1000, or by 2000 um.

In the embodiment illustrated in FIG. 1A, the spectral imagingmicroscope 12 and/or the illumination optical assembly 18 can alsoinclude a reflection illumination optical assembly 18R (illustrated moreclearly in FIG. 1C) for directing the illumination beam 16 at the sample10 when operating in reflection mode. In one embodiment, the reflectionillumination optical assembly 18R includes one or more optical,refractive elements, e.g., lenses and/or windows that direct theillumination beam 16 at the sample 10. In this embodiment, therefractive elements can be operable in the MIR range. Moreover, asdescribed in greater detail herein below, pursuant to the teachings ofthe present invention, the refractive elements can have thicknesses andspacing (i.e. separation) that inhibit wavelength-dependent noise, e.g.,parasitic etalon modulations, from adversely impacting the image qualityand optical spectral resolution of the spectra generated from the set ofwavelength dependent spectral images 13A, 13B, 13C of the sample 10 thatare being generated

Additionally, in certain embodiments, the reflection illuminationoptical assembly 18R can be used to transform, i.e. to increase(magnify) or decrease, the size and profile of the illumination beam 16to match a desired reflection illuminated area on the sample 10. Statedanother way, the reflection illumination optical assembly 18R can beused to condition and focus the illumination beam 16 so that theillumination beam 16 has the desired beam profile on the sample 10. Asnon-exclusive examples, the desired reflection illuminated area isapproximately a circular area bounded by a diameter that isapproximately 50, 100, 200, 250, 500, 600, 650, 700, 1000, or by 2000um.

As noted above, the beam steerer assembly 20 is utilized to steer theillumination beam 16 such that the illumination beam 16 can bealternatively utilized in transmission mode or reflection mode. Thedesign of the beam steerer assembly 20 can be varied. In one embodiment,the beam steerer assembly 20 includes a plurality of beam steerers 20T,20R1, 20R2, e.g. mirrors (reflective in the desired optical frequencyspectrum), which are positioned so as to redirect the path of theillumination beam 16 by approximately ninety degrees. Alternatively, thebeam steerer assembly 20 can have a different design and/or the beamsteerers 20T, 20R1, 20R2 can be positioned so as to redirect the path ofthe illumination beam 16 by greater than or less than approximatelyninety degrees. Still alternatively, the beam steerers 20T, 20R1, 20R2can include a curved mirror that conditions the illumination beam 16 (i)to complement the illumination optical assembly 18, or (ii) to allow forthe elimination of a portion or all of the illumination optical assembly18. Furthermore, the beam steerer assembly may also include one or moreelectrically controllable angular adjustments.

For example, in the embodiment illustrated in FIG. 1A, when utilized intransmission mode, the illumination beam 16 only impinges on a singletransmission beam steerer 20T before being directed toward the sample10. Additionally and/or alternatively, in this embodiment, when utilizedin reflection mode, the illumination beam impinges on two reflectionbeam steerers, i.e. a first reflection beam steerer 20R1 and a secondreflection beam steerer 20R2, before being directed toward the sample10.

It should be appreciated that, in this embodiment, the first reflectionbeam steerer 20R1, which is positioned between the optical source 14 andthe transmission beam steerer 20T, includes a steerer mover 20M that canbe controlled to selectively move the first reflection beam steerer 20R1out of the way of the illumination beam 16. With such design, when thespectral imaging device 12 is being used in transmission mode, the firstreflection beam steerer 20R1 can be selectively moved out of the beampath so that the illumination beam 16 does not impinge on the firstreflection beam steerer 20R1.

The illumination switch 22 enables the spectral imaging microscope 12 toselectively switch between transmission mode and reflection mode. Inparticular, in this embodiment, the illumination switch 22 can beutilized to selectively activate the steerer mover 20M to move the firstreflection beam steerer 20R1 out of the path of the illumination beam16, i.e. when the spectral imaging microscope 12 is being utilized intransmission mode; or to move the first reflection beam steerer 20R1into the path of the illumination beam 16, i.e. when the spectralimaging microscope 12 is being utilized in reflection mode.

Moreover, in reflection mode, as illustrated in FIG. 1A, theillumination beam 16 is directed at the sample 10 with the beam splitter26. The design of the beam splitter 26 can be varied to suit thespecific requirements of the spectral imaging microscope 12. In certainembodiments, the beam splitter 26, e.g., a fifty percent (50%) beamsplitter, can redirect a first portion of the illumination beam 16toward the sample 10, and transmit a second portion (not shown) of theillumination rays 16A of the illumination beam 16. The second portion ofthe illumination beam 16 is subsequently directed away from the systemand not used by the spectral imaging microscope 12. It should be notedthat the second (or discarded) portion of the illumination beam 16 thatis generated from this first pass through the beam splitter 26 is notshown in FIG. 1A for purposes of clarity.

In certain embodiments, the beam splitter 26 can be made from a varietyof infrared transmissive materials, such as ZnSe or Ge, or othermaterials. Additionally, the beam splitter 26 can be a plano-piano beamsplitter, with one side anti-reflection (AR) coated, and the othercoated or uncoated for partial reflectivity. The beam splitter 26 canalso provide lensing action for transforming the illumination beam 16 asdesired. The beam splitter 26 can also incorporate design elements toeliminate first and second surface interference effects due to thecoherent nature of the illumination beam 16. As non-exclusive examples,design elements that would reduce the surface interference effectsinclude anti-reflective coatings (for the optical frequency of thebeam), wedged elements, and/or curved optical surfaces.

The imaging optical assembly 24 can have any suitable design dependingon the specific requirements of the spectral imaging microscope 12. Whenthe illumination rays 16A of the illumination beam 16 are illuminatingthe sample 10 in transmission mode, at least a portion of thetransmitted rays 16T that are transmitted through the sample 10 arereceived by the imaging optical assembly 24 and imaged on the imagesensor 28. Somewhat similarly, when the illumination rays 16A of theillumination beam 16 are illuminating the sample 10 in reflection mode,at least a portion of the reflected rays 16R that are reflected from thesample 10 are received by the imaging optical assembly 24 and imaged onthe image sensor 28. Stated in another fashion, the imaging opticalassembly 24 receives at least a portion of the transmitted rays 16T thatare transmitted through the sample 10, or at least a portion of thereflected rays 16R that are reflected from the sample 10 and forms animage on the image sensor 28.

As utilized herein, the term “imaged rays” 16I shall mean thetransmitted rays 16T or the reflected rays 16R that are collected by theimaging optical assembly 24 and imaged on the image sensor 28. Asprovided herein, the imaging optical assembly 24 receives the imagedrays 16I from a plurality of points on the sample 10 and forms the imageon the image sensor 28.

In one embodiment, the imaging optical assembly 24 can include a firstrefractive element 24A and a second refractive element 24B thatcooperate to form an image of the sample 10 on the image sensor 28.Alternatively, the imaging optical assembly 24 can include greater thantwo refractive elements or only one refractive element.

In one embodiment, the first refractive element 24A can be an objectivelens that collects the imaged rays 16I, and focuses the imaged rays 16Ion the image sensor 28. Moreover, as illustrated, the first refractiveelement 24A is positioned substantially between the sample 10 and thesecond refractive element 24B. Additionally, in one embodiment, thesecond refractive element 24B can be a projection lens that projects theimaged rays 16I toward the image sensor 28. Moreover, as illustrated,the second refractive element 24B is positioned substantially betweenthe first refractive element 24A and the image sensor 28. Further, incertain embodiments, each of the refractive elements 24A, 24B can berefractive in the MIR range and/or the optical frequency of theillumination beam 16. Still further, one or both of the refractiveelements 24A, 24B can be a compound lens. Moreover, as described ingreater detail herein below, pursuant to the teachings of the presentinvention, the refractive elements 24A, 24B can have thicknesses andspacing (i.e. separation) that inhibit wavelength-dependent noise, e.g.,parasitic etalon modulations, from adversely impacting the image qualityand optical spectral resolution of the spectra generated from the set ofwavelength dependent spectral images 13A, 13B, 13C of the sample 10 thatare being generated.

In one embodiment, each refractive element in the spectral imagingdevice 12 has an element optical thickness, t, that is defined by eithert≥1/(2nΔv) or t≤1/(2n(v₂−v₁)); and the spacing (separation distance, d)between adjacent refractive elements is defined by either d≥1/(2nΔv) ord≤1/(2n(v₂−v₁)); where n is refractive index of the respectiverefractive element, Δv is the desired spectral resolution, v₁ is a lowerbound of the desired tuning range, and v₂ is an upper bound of thedesired tuning range. Alternatively, each refractive element is definedby both t≥1/(2nΔv) or t≤1/(2n(v₂−v₁)); and the spacing (separationdistance, d) is defined by both d≥1/(2nΔv) or d≤1/(2n(v₂−v₁)).

It should be appreciated that the fluid, e.g., air or another suitablefluid that fills the spacing between the refractive elements 24A, 24B,and the spacing between the refractive elements 24A, 24B and the imagesensor 28 also function as optical elements that can be refractive inthe MIR range.

Each of the refractive elements 24A, 24B in the spectral imaging device12 is operative in the desired tuning range of the spectral imagingdevice 12 and can be types such as plano-convex, plano-concave,meniscus, and aspherical, as well as other types. For refractive lensesin the MIR range, materials such as Ge, ZnSe, ZnS, Si, CaF, BaF orchalcogenide glass and other materials can be employed. Reflectivelenses can be elliptical, paraboloid, or other shapes. The reflectivesurface can be dichroic coating, Au, Ag, or other surface types.

Further, as shown in the embodiment illustrated in FIG. 1A, the imagedrays 16I, i.e. the transmitted rays 16T or the reflected rays 16R, thatare collected and focused by the first refractive element 24A and thesecond refractive element 24B of the imaging optical assembly 24 aredirected at the beam splitter 26. In this embodiment, if the beamsplitter 26 is a fifty percent (50%) beam splitter, the transmitted rays16T or the reflected rays 16R can be split into (i) the imaged rays 16Ithat are imaged on the image sensor 28, and (ii) discarded rays that aredirected away from the image sensor 28.

It should be further appreciated that when the spectral imaging device12 is being utilized in transmission mode, the illumination switch 22can further activate a splitter mover 26M that moves the beam splitter26 out of the way (out of the beam path) of the transmitted rays 16T, asthe beam splitter 26 is not necessary for directing the illuminationbeam 16 toward the sample 10 (such as is required in the reflection modein this embodiment).

In various embodiments, the image sensor 28 can include atwo-dimensional array of sensors that are used to construct atwo-dimensional image including the two dimensional array of data (dataat each pixel). Additionally, the design of the image sensor 28 can bevaried to correspond to the optical frequency range of the illuminationbeam 16, i.e. of the imaged rays 16I. For example, for a MIR beam 16,the image sensor 28 can be an infrared camera that includes an imagesensor that senses infrared light and converts the infrared light intoan array of electronic signals that represents an image of the sample.Stated in another fashion, if the illumination beam 16 is in the MIRrange, the image sensor 28 can be a MIR imager. More specifically, ifthe illumination beam 16 is in the infrared spectral region from two totwenty μm, the image sensor 28 is sensitive to the infrared spectralregion from two to twenty μm.

Non-exclusive examples of suitable infrared image sensors 28 include (i)vanadium oxide (VOX) and amorphous silicon microbolometer arrays such asthe FPA in the FLIR Tau 640 infrared camera that are typicallyresponsive in the seven to fourteen μm spectral range; (ii) mercurycadmium telluride (HgCdTe or MCT) arrays such as those in the FLIR OrionSC7000 Series cameras that are responsive in the 7.7 to 11.5 μm spectralrange; (iii) indium antimonide (InSb) arrays such as those in the FLIROrion SC7000 Series cameras that are responsive in the 1.5 to 5.5 μmspectral range; (iv) indium gallium arsenide (InGaAs); (v) uncooledhybrid arrays involving VO_(x) and other materials from DRS that areresponsive in the two to twenty μm spectral range; or (vi) any othertype of image sensor that is designed to be sensitive to infrared lightin the two to twenty μm range and has electronics allowing reading outof each element's signal level to generate a two-dimensional array ofimage information (data).

In one specific embodiment, the image sensor 28 is a microbolometer thatincludes a two-dimensional array of photosensitive elements (pixels)that are sensitive to the optical frequency of the illumination beam 16.Stated in another fashion, in one embodiment, the image sensor 28 is amicro-electromechanical systems (MEMS) device fabricated in such a wayas to create a plurality of small bolometer pixel elements that isthermally isolated from the underlying substrate. The spacing betweenthe pixel elements is referred to as the pitch of the array. Asnon-exclusive examples, the two-dimensional array can includeapproximately 640×480; 320×240; 480×480; 80×60; 1080×720; 120×120;240×240; or 480×640 pixels. It should be noted that the information fromthe pixels can be used to generate the output images 13A, 13B, 13Cand/or the spectral cube 13.

During use of the spectral imaging device 12, it is desired to improvethe spectral resolution and quality of the two-dimensional data ofimages of the sample 10 and the spectral cube. More specifically, invarious applications, it is desired to inhibit various noise sourcesfrom adversely impacting the quality of the two-dimensional data of theimages 13A, 13B, 13C of the sample 10 that are being generated. Statedin another manner, in such applications, it is desired to improve thesignal-to-noise ratio (SNR) of the ratioed images of the sample 10.

Unfortunately, in real systems, various random and systematic noisesources may exist which can cause a diminished and/or undesired SNR.Examples of random noise sources include, but are not limited to,quantum (Shot) and thermal (Johnson) noise in the image sensor 28,amplitude and frequency fluctuations of the illumination source, andrandom fluctuations in the transmittance of components contained withinthe spectral imaging device 12. Examples of systematic noise sourcesinclude, but are not limited to, the drift in illumination intensity,frequency, and the directional pointing of the source between trials.

An additional wavelength-dependent noise source in spectroscopic imagingsystems can arise as a result from multiple reflections from surfacesand edges of the refractive elements within the spectral imaging device12. For spectral imaging devices 12 which employ temporally coherentoptical sources 14 such as a laser or optically filtered broad bandsources, the complex electric fields of the multiple reflected beamswill add coherently to produce an optical frequency dependenttransmittance as a result of constructive and destructive interference.

FIG. 2 is an illustration of a simplified spectral imaging device 212that facilitates the discussion of examples of situations where suchmultiple reflections can adversely impact the quality of the images (notshown in FIG. 2) being generated of the sample 10 with an image sensor228. More particularly, FIG. 2 is a simplified schematic illustration ofreflections 215 being generated when an illumination beam 216 from anoptical source 214 is transmitted through refractive elements. Oneexample is that of a transparent window 242 having a physical thicknessof “t” along the beam path and two surfaces each having a small butfinite reflectance, R. Typically these surfaces are coated withanti-reflection coatings which are designed to minimize the magnitude ofreflectance for all optical frequencies contained within the instrumentmeasurement band. However, it is difficult in practice to achievesurface reflectivity values that are sufficiently low across the entiretuning range of the optical source without increasing the cost of thesystem substantially and therefore some amount of reflectivity must betolerated. As a result of the small but finite residual reflectivity ofthe component surfaces, multiple reflections will add up on the imageplane of the image sensing device 228 to produce an opticalfrequency-dependent intensity modulation. In this case, the window 242,e.g. refractive element, acts as a parasitic Fabry-Perot etalon (FPE).

In another example, refractive elements such as lenses 244 having curvedsurfaces and finite thicknesses and separation distances betweenelements, may also act as sources of multiple reflections 215 and as aconsequence produce undesired optical frequency dependent intensitymodulation. Though the exact optical frequency-dependent intensitymodulation characteristic for lenses 244 differs from that of theFabry-Perot etalon, the general principle of the Fabry-Perot etaloncaptures the essence of the physical principles.

It should be appreciated that the examples illustrated in FIG. 2 arenon-exclusive, and that reflections can occur as a result of any of therefractive elements in the path of the illumination beam 16 includingair gap spacings between refractive elements.

For the embodiment illustrated in FIG. 2, the transmittance of an etalondepends on the optical frequency (or wavelength) of the illuminationbeam 216, the index of refraction of the material (n) of the refractiveelement 242, the angle of incidence (θ) of the illumination beam 216 onthe refractive element 242, the surface reflectance (R) of therefractive element 242, and the physical thickness (t) of refractiveelement 242 along the beam path. Since the thickness and index ofrefraction are temperature dependent, the transmittance therefore alsodepends on the ambient temperature.

The optical frequency dependence of a Fabry-Perot etalon can bedescribed using the Airy function as follows:

$\begin{matrix}{T = {\frac{T_{\max}}{1 + {F \cdot {\sin^{2}\left( {\delta/2} \right)}}}.}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

In Equation (1) and elsewhere, (i) T is the transmittance; (ii) T_(max)is maximum transmittance; (iii) F is Finesse; and (iv) δ is the Airyfunction argument. T_(max) can be calculated as provided below:

$\begin{matrix}{T_{\max} = {\left( {1 - \frac{A}{1 - R}} \right)^{2}.}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

In Equation (2) and elsewhere, (i) A is the absorbance of the refractiveelement and (ii) R is the surface reflectance.

The Finesse, F can be calculated as follows:

$\begin{matrix}{F = {\frac{4R}{\left( {1 - R} \right)^{2}}.}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

Further, the Airy function argument δ can be calculated as follows:

δ=2β.   Equation (4)

In Equation (4) and elsewhere, β is a parameter that can be calculatedas follows:

$\begin{matrix}{\beta = {\frac{2\; \pi}{\lambda}\Lambda \; {{\cos (\theta)}.}}} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

In Equation (5) and elsewhere, λ is the optical frequency of theillumination beam, and Λ is the optical thickness of the refractiveelement. The optical thickness Λ and can be calculated as follows:

Λ=n·t.   Equation (6)

In Equation (6) and elsewhere, n is the index of refraction of therefractive element and t is the physical thickness of the refractiveelement. Thus, the optical thickness of the material, Λ, (optical pathlength through element) is calculated by the product of the index ofrefraction, n, and the physical thickness of the transparent material, tof the refractive element.

Further, an optical frequency period of modulation Δv_(mod) of thetransmittance function can be expressed in units of wavenumbers (cm⁻¹)asfollows:

$\begin{matrix}{{\Delta \; v_{mod}} = {\frac{1}{2\; \Lambda}.}} & {{Equation}\mspace{14mu} (7)}\end{matrix}$

Thus, the modulation of the transmittance is periodic in opticalfrequency space and is given by the reciprocal of twice the opticalthickness of the material, Λ. Further, the strength of the modulationdepends on the reflectivity R of the surfaces and in the range of smallvalues of R (<5%), the peak-to-peak modulation is approximately fourtimes that of the value of surface reflectance. Therefore, for arefractive element, e.g. window, having a reflectance of 2.5%, thepeak-to-peak modulation will be approximately 10% and would limit theSNR to about 10:1. As an approximation, the modulation of a refractiveelement can be estimated by treating it as a Fabry-Perot etalon with anequivalent thickness (t) given by its center thickness.

FIG. 3 is a simplified graph that illustrates transmittance versuswavenumber for refractive elements having three alternativereflectances, R. Curve 302 (short dashed line) plots the transmittanceversus wavenumber for a refractive element having a reflectance of 0.1(R=0.1); Curve 304 (long dashed line) plots the transmittance versuswavenumber for a refractive element having a reflectance of 0.4 (R=0.4);and Curve 306 (solid line) plots the transmittance versus wavenumber fora refractive element having a reflectance of 0.9 (R=0.9).

In FIG. 3, the optical frequency period of the modulation of thetransmittance Δv_(mod) is equal to ten cm⁻¹ wavenumbers. However, theamount of change in the transmittance varies significantly based on thereflectivity R.

As provided herein, if the background normalization is not performed,the transmittance modulation associated with this refractive elementwill directly corrupt the spectral data SNR and produce undesirableartifacts in the images. One way to mitigate this effect is to ratio thespectral cube with a background spectral cube. This approach iseffective if the optical frequency dependent component modulation andthe source optical frequency are highly repeatable from run-to-run.However, in practice, the optical thickness of the parasitic etalons ofthe system will vary by a small amount due to changes in theenvironmental temperature, pressure, or stress of the system so as tocause small but significant changes in the transmittance function whoseexact dependence on time may not be known a priori. Also, the opticalfrequency of the coherent light source may vary from run-to-run becauseof stochastic laser dynamics or imperfections in the tuning mechanisms.These small differences in either the source optical frequency or theparasitic etalon modulation function will result in changes in thetransmittance value and therefore limit the SNR of the ratioed imagescaptured by the system. Therefore, further techniques are required toimprove the SNR of the system.

FIG. 4A illustrates an ideal image 402 of the sample that would becaptured by the image sensor if all of the noise is eliminated from thespectral imaging device. However, each refractive element will createnoise in the captured image. As provided herein, for a given material,reflectance and optical frequency, a thicker refractive element producesa high-Fourier space frequency spectral artifact in the image cube,whereas a thinner refractive element produces a low-Fourier spacefrequency spectral artifact in the image. FIG. 4B illustrates a firstnon-ideal image 404 (the image from FIG. 4A plus “N's” that representnoise) of the sample that would be captured by the image sensor if athin refractive element is in the optical path. FIG. 4B illustrates theconcept that if the thin element is used in the system, the resultingfirst non-ideal image 404 will be somewhat equivalent to the ideal image402 plus the spectral noise introduced as a result of the thinrefractive element (thin parasitic etalon).

FIG. 4C illustrates a second non-ideal image 406 (the image from FIG. 4Aplus “N's” that represent noise) of the sample that would be captured bythe image sensor if a thick refractive element is in the optical path.FIG. 4C illustrates the concept that if the thick element is used in thesystem, the resulting second non-ideal image 406 will be somewhatequivalent to the ideal image 402 plus the spectral noise introduced asa result of the thick refractive element (thick parasitic etalon).

FIG. 5A is a graph of transmittance versus wavenumber in the opticalfrequency space, that includes a first curve 500 (solid line) thatillustrates the transmittance modulation of a refractive element havingan optical thickness of 0.5 millimeters (Λ=0.5 mm.), and a second curve502 (short dashes) that illustrates the transmittance modulation of arefractive element having an optical thickness of 4 millimeters (Ε=4mm.). In this example, the reflectivity (2.5%) and material are the samefor both refractive elements. The only difference is the opticalthickness. Comparing curve 500 to curve 502, in the optical frequencyspace, the transmittance modulation has a longer period between maximafor the thinner element than for the thicker element.

A useful way to analyze the effects of the parasitic etalon caused bythe refractive elements (e.g. lens and other elements in the opticalpath) of the spectral imaging device is to examine the optical frequencymodulation function in the reciprocal Fourier frequency space havingunits of inverse wavenumber of centimeter (cm). This can be accomplishedby applying a Fourier transform to the modulation transfer function ofthe etalon from FIG. 5A. More specifically, FIG. 5B is a graph thatillustrates the Fourier space power spectral density (“PSD”) versusFourier space frequency in the Fourier Space, that includes a firstcurve 504 (solid line) that illustrates the transmittance modulation ofthe refractive element having the optical thickness of 0.5 millimeters(Λ=0.5 mm.), and a second curve 506 (short dashes) that illustrates thetransmittance modulation of the refractive element having an opticalthickness of 4 millimeters (Λ=4 mm.). Again, in this example, thereflectivity (2.5%) and material are the same for both refractiveelements. The only difference is the optical thickness.

Comparing curve 504 to curve 506, in the Fourier space frequency, thetransmittance modulation of the thinner refractive element isconcentrated near zero centimeters, and transmittance modulation of thethicker refractive element is spread out along the Fourier space and notconcentrated near zero centimeters. Further, when the refractive elementis optically thinner (0.5 mm versus 4 mm), the Fourier frequency spacecomponents of the parasitic etalon are lower.

As provided above, the spectral imaging device 12 includes a Fourierspace measurement pass band 508 (also referred to as the “pass band”)which is the reciprocal of the desired spectral resolution. In oneembodiment, the upper limit A, and lower limit B, of the pass band 508are given by A=1/(2Δv), and B=−1/(2Δv), where Δv, is the desiredspectral resolution that the spectral imaging device is designed toachieve. For example, in this non-exclusive example, the desiredspectral resolution is 4 cm⁻¹ (Δv=4 cm⁻¹). The Fourier measurement passband for this non-exclusive embodiment would therefore have an upperlimit A, and the lower limit B of the pass band 508 in Fourier space0.125 cm (2 cm⁻¹) and −0.125 cm (−2 cm⁻¹), respectively.

As illustrated in FIG. 5B, the Fourier space representation of themodulation shows that the thinner etalon produces non-dc componentswithin a typical measurement pass band of 0.25 cm.

With reference to both FIGS. 5A and 5B, the thinner refractive element(Λ=0.5 mm) has Fourier components (curve 504) that fall squarely withinthe pass band 508, whereas many of the Fourier components (curve 506) ofthe thicker refractive element (Λ=4 mm) fall outside of the pass band508.

FIG. 6A is a simplified illustration of the optical frequency space 600for an spectral imaging device. FIG. 6A also illustrates the spectralmeasurement range 607, the desired spectral resolution Δv; the lowerbound of the spectral band v₁; and the upper bound of the spectral band,v₂.

FIG. 6B is a simplified illustration of the corresponding Fourier space602 for the spectral imaging device, including the pass band 608. Asprovided herein, the pass band 608 includes a gap 610 in the pass band608 that exists near the origin of Fourier space 602 due to the finiteoptical frequency range of the spectral imaging system. Because of thegap 610, the pass band 608 includes a negative Fourier space partition612 and a positive Fourier space partition 614 that are spaced apart andseparated by the gap 610.

This gap 610 has upper limit C, determined with equation C=1/(2(v₂−v₁))and a lower limit D, determined with equation D=−1/(2(v₂−v₁)), where v₂and v₁ are the upper and lower bounds of the optical frequency rangecovered by the spectroscopic imaging system. Further, as provided above,the upper limit A, and lower limit B, of the pass band 508 are given byA=1/(2Δv), and B=−1/(2Δv).

In one, non-exclusive embodiment, for an IR imaging system covering the900-1800 cm⁻¹ spectral range and having a spectral resolution of 4 cm⁻¹,the pass band will have the following four values: A=1.25 mm; B=−0.125mm; C=+5.55 um; and D=−5.5 um.

As provided herein, the architecture of the spectral imaging device canbe adjusted and designed so that parasitic etalon modulation Fourierspace components fall outside of the negative Fourier space partition612 and the positive Fourier space partition 614 of the measurement passband 608. This can be accomplished by designing and positioning therefractive elements in the spectral imaging device so that the opticalthickness of parasitic etalons are outside the negative Fourier spacepartition 612 and the positive Fourier space partition 614.

FIG. 6B illustrates a non-optimized design that includes (i) a Fourierspace component of a first parasitic etalon component 620 (illustratedwith a dashed arrow); (ii) a Fourier space component of a secondparasitic etalon component 622 (illustrated with a dashed arrow); (iii)a Fourier space component of a third parasitic etalon component 624(illustrated with a dashed arrow); and (iv) a Fourier space component ofa fourth parasitic etalon component 626 (illustrated with a dashedarrow). In this non-optimized design, (i) the first and second etaloncomponents 620, 622 are in the negative Fourier space partition 612 ofthe pass band 608; and (ii) the third and fourth etalon components 624,626 are in the positive Fourier space partition 614 of the pass band608.

As provided herein, the architecture of the spectral imaging device canbe adjusted and designed to move the (i) the first and second etaloncomponents 620, 622 out of the negative Fourier space partition 612 ofthe pass band 608; and (ii) the third and fourth etalon components 624,626 out of the positive Fourier space partition 614 of the pass band608. More specifically, the architecture of the spectral imaging devicecan be adjusted to shift and move (i) the first etalon component 620 outthe negative Fourier space partition 612 of the pass band 608 asillustrated by solid arrow 630; (ii) the second etalon component 622 outthe negative Fourier space partition 612 of the pass band 608 asillustrated by solid arrow 632 into the gap 610; (iii) the third etaloncomponent 624 out the positive Fourier space partition 614 of the passband 608 as illustrated by solid arrow 634 into the gap 610; and (iv)the fourth etalon component 626 out the positive Fourier space partition614 of the pass band 608 as illustrated by solid arrow 636.

Stated in another fashion, the architecture of the refractive elementsof the spectral imaging device are (i) sufficiently thick to move theFourier space components of the first and fourth parasitic etaloncomponents 620, 626 higher than the pass band upper and lower limits; or(ii) sufficiently thin so as to push the Fourier space components of thesecond and third parasitic etalon components 622, 624 between thepositive and negative pass band regions. In FIG. 6B, the dashed arrows620, 622, 614, 626 illustrate parasitic etalon components prior toshifting, while the solid line arrows illustrate parasitic etaloncomponents 630, 632, 634, 634 after shifting.

Thus, in certain embodiments, by properly designing the system,parasitic etalon components 620, 622, 624, 626 are shifted out of theoperating pass band by forcing optical path length of parasitic etalonsto be greater than 1/2Δv. A non-exclusive example, of typical parametervalues for a mid-infrared spectroscopic imaging system are Δv=4 cm⁻¹,v1=900 cm⁻¹, v2=1800 cm⁻¹. In this example, in Fourier space, theseparameters create a positive and negative pass band range of +5.56 to+1250 μm and −5.56 to −1250 μm respectively.

Returning back to FIG. 1B, as noted above, FIG. 1B is a simplifiedschematic illustration of a transmission beam path 50 of theillumination beam 16 from the optical source 14 of the spectral imagingmicroscope 12, with the sample 10 being interrogated via transmission ofthe illumination beam 16 through the sample 10. In particular, FIG. 1Billustrates the various components of the spectral imaging microscope 12that are utilized when the sample 10 is being interrogated intransmission mode. It should be appreciated that the additionalcomponents of the spectral imaging microscope 12 that are only utilizedwhen the sample 10 is being interrogated in reflection mode have beenomitted from FIG. 1B for purposes of clarity and ease of description.

As illustrated in FIG. 1B, when being utilized in transmission mode, thecomponents of the spectral imaging microscope 12 include the opticalsource 14 which generates and/or emits the illumination beam 16, thetransmission beam steerer 20T, the image sensor 28, and various opticalelements, e.g., refractive elements, including such refractive elementsthat make up the transmission illumination optical assembly 18T and theimaging optical assembly 24. As utilized herein, such optical elements,including the refractive elements that make up the transmissionillumination optical assembly 18T and the imaging optical assembly 24,can be referred to generally as a “transmission optical assembly”.

More specifically, as shown, the transmission optical assembly caninclude (i) a first refractive element 46A, e.g., a window; (ii) asecond refractive element 46B, e.g., a refractive lens; (iii) a thirdrefractive element 46C, e.g., a window; (iv) a fourth refractive element46D, e.g., including the sample 10 and/or any slide that can be utilizedfor the sample 10; (v) a fifth refractive element 46E, e.g., the firstrefractive lens 24A of the imaging optical assembly 24; (vi) a sixthrefractive element 46F, e.g., the second refractive lens 24B of theimaging optical assembly 24; and (vii) a seventh refractive element 46G,e.g., a window positioned near to and/or in front of the image sensor28. Moreover, each of the refractive elements 46A-46G are spaced apartfrom one another, as well as being spaced apart from the optical source14 and the transmission beam steerer 20T.

As illustrated in this embodiment, when being used in transmission mode,the transmission beam path 50 of the illumination beam 16 follows fromthe optical source 14 to the first refractive element 46A, to the secondrefractive element 46B, to the transmission beam steerer 20T, to thethird refractive element 46C, to the fourth refractive element 46D(including the sample 10), to the fifth refractive element 46E, to thesixth refractive element 46F, to the seventh refractive element 46G, andfinally to the image sensor 28.

Additionally, as provided herein, the thickness of each of therefractive elements 46A-46G, as well as the spacing (also referred toherein as a “separation distance”) between each of the components isspecifically designed such that Fourier space components of thetransmittance function for each of the refractive elements 46A-46G andeach of the separation distances fall outside the measurement pass band.For example, (i) the first refractive element 46A has a first elementoptical thickness, t₁; (ii) the second refractive element 46B has asecond element optical thickness, t₂; (iii) the third refractive element46C has a third element optical thickness, t₃; (iv) the fourthrefractive element 46D has a fourth element optical thickness, t₄; (v)the fifth refractive element 46E has a fifth element optical thickness,t₅; (vi) the sixth refractive element 46F has a sixth element opticalthickness, t₆; and (vii) the seventh refractive element 46G has aseventh element optical thickness, t₇, which are each designed to have aFourier space component of the transmittance function that falls outsidethe measurement pass band.

Further, (i) a first separation distance, d₁, between the optical source14 and the first refractive element 46A; (ii) a second separationdistance, d₂, between the first refractive element 46A and the secondrefractive element 46B; (iii) a third separation distance, d₃, betweenthe second refractive element 46B and the transmission beam steerer 20T;(iv) a fourth separation distance, d₄, between the transmission beamsteerer 20T and the third refractive element 46C; (v) a fifth separationdistance, d₅, between the third refractive element 46C and the fourthrefractive element 46D; (vi) a sixth separation distance, d₆, betweenthe fourth refractive element 46D and the fifth refractive element 46E;(vii) a seventh separation distance, d₇, between the fifth refractiveelement 46E and the sixth refractive element 46F; and (viii) an eighthseparation distance, d₈, between the sixth refractive element 46F andthe seventh refractive element 46G, are also each designed to have aFourier space component of the transmittance function that falls outsidethe measurement pass band.

Additionally, as noted above, FIG. 1C is a simplified schematicillustration of a reflection beam path 52 of the illumination beam 16from the optical source 14 of the spectral imaging microscope 12, withthe sample 10 being interrogated via reflection of the illumination beam16 off of the sample 10. In particular, FIG. 1C illustrates the variouscomponents of the spectral imaging microscope 12 that are utilized whenthe sample 10 is being interrogated in reflection mode. It should beappreciated that the additional components of the spectral imagingmicroscope 12 that are only utilized when the sample 10 is beinginterrogated in transmission mode have been omitted from FIG. 1C forpurposes of clarity and ease of description.

As illustrated in FIG. 1C, when being utilized in reflection mode, thecomponents of the spectral imaging microscope 12 include the opticalsource 14 which generates and/or emits the illumination beam 16, thereflection beam steerers 20R1, 20R2, the image sensor 28, and variousoptical elements, e.g., refractive elements, including the beam splitter26 and such refractive elements that make up the reflection illuminationoptical assembly 18R and the imaging optical assembly 24. As utilizedherein, such optical elements, including the beam splitter 26 and therefractive elements that make up the reflection illumination opticalassembly 18R and the imaging optical assembly 24, can be referred togenerally as a “reflection optical assembly”.

More specifically, as shown, the reflection optical assembly can include(i) the first refractive element 46A, e.g., a window; (ii) an eighthrefractive element 46H, e.g., a refractive lens; (iii) a ninthrefractive element 46I, e.g., the beam splitter 26; (iv) the fourthrefractive element 46D, e.g., including the sample 10 and/or any slidethat can be utilized for the sample 10; (v) the fifth refractive element46E, e.g., the first refractive lens 24A of the imaging optical assembly24; (vi) the sixth refractive element 46F, e.g., the second refractivelens 24B of the imaging optical assembly 24; and (vii) the seventhrefractive element 46G, e.g., a window positioned near to and/or infront of the image sensor 28. Moreover, each of the refractive elements46A, 46D-46I are spaced apart from one another, as well as being spacedapart from the optical source 14 and the reflection beam steerers 20R1,20R2.

As illustrated in this embodiment, when being used in reflection mode,the reflection beam path 52 of the illumination beam 16 follows from theoptical source 14 to the first refractive element 46A, to the firstreflection beam steerer 20R1, to the second reflection beam steerer20R2, to the eighth refractive element 46H, to the ninth refractiveelement 46I (the beam steerer 26), to the sixth refractive element 46F,to the fifth refractive element 46E, to the fourth refractive element46D (including the sample 10), back to the fifth refractive element 46E,to the sixth refractive element 46F, to the ninth refractive element 46I(the beam splitter 26), to the seventh refractive element 46G, andfinally to the image sensor 28.

Additionally, as provided herein, the thickness of each of therefractive elements 46A, 46D-46I, as well as the spacing (i.e. the“separation distance”) between each of the components is specificallydesigned such that Fourier space components of the transmittancefunction for each of the refractive elements 46A, 46D-46I and each ofthe separation distances fall outside the measurement pass band. Forexample, (i) the first refractive element 46A has the first elementoptical thickness, t₁; (ii) the fourth refractive element 46D has thefourth element optical thickness, t₄; (iii) the fifth refractive element46E has the fifth element optical thickness, t₅; (iv) the sixthrefractive element 46F has the sixth element optical thickness, t₆; (v)the seventh refractive element 46G has the seventh element opticalthickness, t₇, (vi) the eighth refractive element 46H has an eighthelement optical thickness, t₈; and (iii) the ninth refractive element46I has a ninth element optical thickness, t₉, which are each designedto have a Fourier space component of the transmittance function thatfalls outside the measurement pass band.

Further, (i) the first separation distance, d₁, between the opticalsource 14 and the first refractive element 46A; (ii) a ninth separationdistance, d₉, between the first refractive element 46A and the firstreflection beam steerer 20R1; (iii) a tenth separation distance, d₁₀,between the first reflection beam steerer 20R1 and the second reflectionbeam steerer 20R2; (iv) an eleventh separation distance, d₁₁, betweensecond reflection beam steerer 20R2 and the eighth refractive element46H; (v) a twelfth separation distance, d₁₂, between the eighthrefractive element 46H and the ninth refractive element 46I; (vi) thesixth separation distance, d₆, between the fourth refractive element 46Dand the fifth refractive element 46E; (vii) the seventh separationdistance, d₇, between the fifth refractive element 46E and the sixthrefractive element 46F; and (viii) a thirteenth separation distance,d₁₃, between the ninth refractive element 46I and the seventh refractiveelement 46G, are also each designed to have a Fourier space component ofthe transmittance function that falls outside the measurement pass band.

Additionally, as provided herein, in certain embodiments, the position(i.e. spacing) of the components and the design (i.e. thickness) of thevarious components in the spectral imaging device 12 can be adjusted anddesigned to insure that parasitic etalon modulation occurs outside ofthe pass band.

Table 1, shown below, provides one, non-exclusive example, of possibleelement-to-element separation distances (“d₁” through “d₁₄”) and elementoptical thicknesses (“t₁” through “t₉”) which meet design criteria forthe spectral imaging microscope 12. It should be noted that the numbersin Table 1 are based on the spectral imaging microscope 12 beingdesigned to provide a 4 cm⁻¹ spectral resolution in each of theembodiments, i.e. in each of the transmission mode and the reflectionmode. Additionally, it should be noted that the separation distances andelement optical thicknesses may need to be different than thosespecifically provided in Table 1 to desirably manage the parasiticetalon components, if the design and characteristics of the spectralimaging microscope 12 are changed.

TABLE 1 Distance/ Thickness Minimum Optimum Typical range of ParameterDesign Criteria Design Criteria values in practice d₁ >1.25 mm >10 mm1.25-500 mm d₂ >1.25 mm >10 mm 1.25-500 mm d₃ >1.25 mm >10 mm 1.25-500mm d₄ >1.25 mm >10 mm 1.25-500 mm d₅ >1.25 mm >10 mm 1.25-500 mmd₆ >1.25 mm >10 mm 1.25-500 mm d₇ >1.25 mm >10 mm 1.25-500 mm d₈ >1.25mm >10 mm 1.25-500 mm d₉ >1.25 mm >10 mm 1.25-500 mm d₁₀ >1.25 mm >10 mm1.25-500 mm d₁₁ >1.25 mm >10 mm 1.25-500 mm d₁₂ >1.25 mm >10 mm 1.25-500mm d₁₃ >1.25 mm >10 mm 1.25-500 mm d₁₄ >1.25 mm >10 mm 1.25-500 mmt₁ >(1.25 mm)/n >(10 mm)/n 1-5 mm (n = 1.3-4.3) t₂ >(1.25 mm)/n >(10mm)/n 1-5 mm (n = 1.3-4.3) t₃ >(1.25 mm)/n >(10 mm)/n 1-5 mm (n =1.3-4.3) T₄ >(1.25 mm)/n >(10 mm)/n 1-5 mm (n = 1.3-4.3) t₅ >(1.25mm)/n >(10 mm)/n 1-5 mm (n = 1.3-4.3) t₆ >(1.25 mm)/n >(10 mm)/n 1-5 mm(n = 1.3-4.3) t₇ >(1.25 mm)/n >(10 mm)/n 1-5 mm (n = 1.3-4.3) t₈ >(1.25mm)/n >(10 mm)/n 1-5 mm (n = 1.3-4.3) t₉ >(1.25 mm)/n >(10 mm)/n 1-5 mm(n = 1.3-4.3)

Additionally, as provided herein, the influence of parasitic etaloncomponents can be reduced and managed in other unique ways. For example,with reference to FIG. 1A, as provided above, the spectral imagingdevice 12 can be controlled to generate a separate output image 13A,13B, 13C at a plurality of distinct target optical frequencies. Withthis design, each target optical frequency includes a correspondingoutput image 13A, 13B, 13C. Subsequently, the control system 30 uses theoutput images 13A, 13B, 13C to generate the spectral cube 13.

Because the noise is optical frequency dependent, as provided herein,for each target optical frequency, the spectral imaging device 12 cancapture a plurality of preliminary images at optical frequencies near orequal to the target optical frequency. Subsequently, for each targetoptical frequency, the corresponding plurality of preliminary images canbe used to generate a corresponding output image having reduced noisefor that target optical frequency.

FIG. 7A is a graph that illustrates optical frequency versus time. Asprovided herein, the control system 30 (illustrated in FIG. 1A) cancontrol the tunable optical source 14 (illustrated in FIG. 1A) togenerate an illumination beam 16 (illustrated in FIG. 1A) having acenter optical frequency that changes in a stepped pattern from a firstoptical frequency to an eleventh optical frequency over time. Somewhatsimilarly, FIG. 7B is a graph that illustrates optical frequency versustime. In this example, the control system 30 (illustrated in FIG. 1A)controls the tunable optical source 14 (illustrated in FIG. 1A) togenerate an illumination beam 16 (illustrated in FIG. 1A) having acenter optical frequency that changes in a linear fashion from a firstoptical frequency to an eleventh optical frequency over time. It shouldbe noted that the optical frequency can be adjusted in another fashionthan illustrated in FIGS. 7A and 7B.

In these examples, the first through eleventh optical frequencies areeach within the desired tuning range of the spectral imaging device 12(illustrated in FIG. 1A). Further, in these simplified examples, (i) attime one the illumination beam 16 has a center optical frequency of one;(ii) at time two the illumination beam 16 has a center optical frequencyof two; (iii) at time three the illumination beam 16 has a centeroptical frequency of three; (iv) at time four the illumination beam 16has a center optical frequency of four; (v) at time five theillumination beam 16 has a center optical frequency of five; (vi) attime six the illumination beam 16 has a center optical frequency of six;(vii) at time seven the illumination beam 16 has a center opticalfrequency of seven; (viii) at time eight the illumination beam 16 has acenter optical frequency of eight; (ix) at time nine the illuminationbeam 16 has a center optical frequency of nine; (x) at time ten theillumination beam 16 has a center optical frequency of ten; and (xi) attime eleven the illumination beam 16 has a center optical frequency ofeleven.

It should be noted that one or more of the optical frequencies can be atarget optical frequency 753A, 753B, 753C. In this non-exclusiveexample, optical frequencies three, six and nine are target opticalfrequencies 753A, 753B, 753C.

FIG. 7C illustrates a plurality of preliminary images 751A, 751B, 751C,751D, 751E, 751F, 751G, 752H, 751I, 751J, 751K that can be used togenerate a separate output image 713A, 713B, 713C for each targetoptical frequency 753A, 753B, 753C (illustrated in FIGS. 7A and 7B)

In this simplified example, with reference to FIGS. 7A-7C, the spectralimaging device 12 (illustrated in FIG. 1A) is controlled to (i) capturea first preliminary (“sampling”) image 751A while illuminating thesample 10 (illustrated in FIG. 1A) with the illumination beam 16(illustrated in FIG. 1A) having the first center optical frequency (attime=1); (ii) capture a second preliminary image 751B while illuminatingthe sample 10 with the illumination beam 16 having the second centeroptical frequency (at time=2); (iii) capture a third preliminary image751C while illuminating the sample 10 with the illumination beam 16having the third center optical frequency (at time=3); (iv) capture afourth preliminary image 751D while illuminating the sample 10 with theillumination beam 16 having the fourth center optical frequency (attime=4); (v) capture a fifth preliminary image 751E while illuminatingthe sample 10 with the illumination beam 16 having the fifth centeroptical frequency (at time=5); (vi) capture a sixth preliminary image751 F while illuminating the sample 10 with the illumination beam 16having the sixth center optical frequency (at time=6); (vii) capture aseventh preliminary image 751G while illuminating the sample 10 with theillumination beam 16 having the seventh center optical frequency (attime=7); (viii) capture an eighth preliminary image 751H whileilluminating the sample 10 with the illumination beam 16 having theeighth center optical frequency (at time=8); (ix) capture a ninthpreliminary image 751I while illuminating the sample 10 with theillumination beam 16 having the ninth center optical frequency (attime=9); (x) capture a tenth preliminary image 751J while illuminatingthe sample 10 with the illumination beam 16 having the tenth centeroptical frequency (at time=10); and (xi) capture an eleventh preliminaryimage 751K while illuminating the sample 10 with the illumination beam16 having the eleventh center optical frequency (at time=11).

Subsequently, the spectral imaging device 12 uses one or more of thepreliminary (“sampling”) images 751A-751K to generate the separatetarget output image 713A, 713B, 713C for each target optical frequency753A, 753B, 753C. The number of preliminary images 751A-751K used togenerate the separate output images 713A, 713B, 713C can vary. Asnon-exclusive examples, 2, 3, 4, 5, 6, 7 or 8 preliminary images751A-751K can be used to generate each of the output images 713A, 713B,713C. Typically, the preliminary images 751A-751K utilized are capturednear or at the target optical frequency.

In one example, if five preliminary images 751A-751K are used, (i) thefirst through fifth preliminary images 751A-751E are used to generatethe output image 713A for target optical frequency 753A at opticalfrequency three; (ii) the fourth through eighth preliminary images751D-751H are used to generate the output image 713B for target opticalfrequency 753B at optical frequency six; and (iii) the seventh througheleventh preliminary images 751G-751K are used to generate the outputimage 713C for target optical frequency 753C at optical frequency nine.

The method used to combine the multiple preliminary images to generatethe respective output images can vary. In one, non-exclusive embodiment,the corresponding multiple preliminary images are passed through alow-pass filter to generate the respective output image. Stated inanother fashion, a low-pass filter is subsequently applied to thespectral response of each pixel in the respective preliminary images tocreate an output spectral image at a lower spectral resolution with lessnoise. In this example, (i) the first through fifth preliminary images751A-751E are passed through a low-pass filter to generate the outputimage 713A for target optical frequency 753A at optical frequency three;(ii) the fourth through eighth preliminary images 751D-751H are passedthrough a low-pass filter to generate the output image 713B for targetoptical frequency 753B at optical frequency six; and (iii) the sevenththrough eleventh preliminary images 751G-751K are passed through alow-pass filter to generate the output image 713C for target opticalfrequency 753C at optical frequency nine.

As non-exclusive examples, the low-pass filter can utilize either arunning average or Gaussian filter, and optionally followed bysub-sampling through decimation. One such method is to perform a simpleaverage of the collected data points. Another method is to perform asimple average of the data points after the extreme values are removedfrom the data set. Extreme values may be defined, for example, as thosefalling outside of a predefined multiple of the root-mean-square of thecollection. Another method is to pass a low-pass filter over the dataset, such as a Chebyshev filter. The low-pass filter may be applied inoptical frequency space or in Fourier frequency space and may beperformed before or after any ratio taken between a data collection andbackground data collection.

It should be noted that a sampling optical frequency sampling period (orinverse of the optical frequency sampling rate) between each of thefirst through eleventh optical frequencies in which preliminary imagesare captured can be varied pursuant to the teachings provided herein. Incertain embodiments, the optical frequency step size is the reciprocalof the sampling rate. In one embodiment, the optical frequency step sizeis sufficiently small such that it does not produce aliasing of theFourier frequency components of the optical frequency dependenttransmittance function of the parasitic etalons contained along the beampath into the measurement pass band. For example, the optical frequencystep size should be less than or equal to the free spectral range (FSR)of the refractive element in the spectral imaging device 12 having theshortest free spectral range of a refractive element in the beam pathdivided by two.

Stated in another fashion, in certain embodiments, for this method to beeffective, the sampling rate must be sufficiently high, and the samplingperiod, Δv_(sampling), sufficiently small, so as to inhibit aliasing ofthe spurious spectral signal into the measurement pass band. As usedherein, the term “sampling rate” shall mean the inverse of the opticalfrequency sampling period, and the term “sampling period” shall meanoptical frequency sampling period Δv_(sampling). Aliasing may cause theFourier frequency components of the spurious spectral signals to shiftfrom outside of the pass band to into the measurement pass band. In sucha case, removal of the spurious signals by filtering can be achieved,but at the expense of sacrificing spectral resolution of the system,which is undesired. As non-exclusive examples, the optical frequencysampling period can be approximately within the range 0.1-10 cm⁻¹, andspecific values of 0.1, 0.25, 0.33, 0.5, 0.67, 0.7, 1.0, 1.5, 2.0, 2.5,3.33, 5.0, and 10 cm⁻¹.

As provided herein, the parasitic etalons have Fourier components whichrepeat at integer multiples of their free-spectral-range (FSR), and isgiven by 1/2 nL in units of wavenumbers. In order to ensure that eachspurious component falls outside of the pass band, the measurementsamples should be collected at interval optical spacing, Δv_(sampling),which are at least as small as half the FSR associated with theparasitic etalon. The FSR should also be smaller than the minimumspectral resolution, Δv, of the system in order that the spectralresolution is not compromised by the filtering of the spurious signal.

Δv _(sampling)≤FSR/2≤Δv.   Equation (8)

Stated in yet another fashion, the control system 30 controls thetunable light source 14 to generate a set of discrete sampling opticalfrequencies near a target optical frequency, with adjacent samplingoptical frequencies of the set being spaced apart a sampling opticalfrequency step, and the sampling optical frequency step beingsufficiently small such that it does not produce aliasing of the Fouriercomponents of the optical frequency dependent transmittance function ofthe parasitic etalons contained along the beam path into the measurementpass band. Further, the control system controls the image sensor tocapture or construct a separate, two dimensional sampling image at eachdiscrete sampling optical frequency, and the control system constructs atarget output image of the sample for the target optical frequency usingthe separate two dimensional sampling images at each discrete samplingoptical frequency.

As provided herein, the term “sampling optical frequency step” shallmean the smallest allowed difference between adjacent sampling opticalfrequencies. In alternative, non-exclusive embodiments, the samplingoptical frequency step can be approximately 0.1, 0.2, 0.25, 0.33, 0.5,0.67, 0.7, 1.0, 2.0, 4.0, 8.0, or 16, wavenumbers. In this example, thetarget optical frequency step (difference between target opticalfrequencies) is larger than the sampling optical frequency step.

In summary, as provided herein, the influence of parasitic etaloncomponents can be reduced and managed by discrete sampling, filtering,and decimation. First, a plurality of preliminary (“sampling”) imagesare captured. Subsequently, the preliminary images are filtered tocreate a lower spectral resolution image that can optionally besub-sampled (e.g. via decimation) to remove the redundant informationfrom now being oversampled. Thus, a collection of spectral images iscaptured at multiple discrete optical frequencies in the neighborhood ofthe desired measurement frequency. This collection of data points isthen mathematically filtered so as to produce a single higher-fidelitydata point.

FIGS. 8A and 8B are useful for the further discussion of the method offiltering the coherent noise caused by a parasitic etalon by means ofdiscrete sampling, filtering, and subsequent decimation. The opticalfrequency sampling period is set by the prescription and is chosen to besufficiently fine, so as not to introduce spectral leakage due toaliasing into the measurement pass band.

More specifically, FIG. 8A is a graph of transmittance versus wavenumberin the optical frequency space that illustrates (i) the modulation 802(illustrated with a dashed line) of the first parasitic etaloncomponents for a first refractive element having a first thickness (e.g.t=4 mm), and (ii) the modulation 804 (illustrated with a solid line) ofthe second parasitic etalon components for a second refractive elementhaving a second thickness (e.g. t=0.5 mm) which is less than the firstthickness. In this example, the first element has an optical path thatis longer than the second element. The graph in FIG. 8A includes (i)circles that represent discrete samplings of the first parasitic etaloncomponents 1012 sampled at 4.1 cm⁻¹, and (ii) solid dots that representdiscrete samplings of the of the second etalon components. As anon-exclusive example, the sampling intervals are 4.1 cm⁻¹.

FIG. 8B is a graph in the Fourier space frequency that illustrates (i)the modulation 806 (illustrated with a solid gray line) of the firstparasitic etalon components of the first refractive element (e.g. t=4mm), and (ii) the modulation 808 (illustrated with a solid black line)of the second parasitic etalon components of the second optical element(e.g. t=0.5 mm). The graph of FIG. 8B also includes (i) discretesamplings 810 (illustrated with a gray dashed line) of the firstparasitic etalon components sampled at 4.1 cm⁻¹, and (ii) discretesampling 812 (illustrated with a black dashed line) of the second etaloncomponents sampled at 4.1 cm⁻¹.

A pass band 814 (e.g. a 0.250 cm passband) is also illustrated in FIG.8B. It should be noted that insufficient sampling at 4.1 cm⁻¹ of thefirst refractive element (t=4 mm) can cause Fourier components of thefirst parasitic etalon components to leak into the pass band 814.

FIG. 9 is a graph in the optical frequency space of a sampled raw signaldata 902 (illustrated with a gray line) and a sampled and filteredsignal data 904 (illustrated with a black line). As can be seen fromFIG. 9, the variation in signal data 902 cause by the parasitic etaloncomponents is greatly reduced by sampling, filtering, and subsequentdecimation as seen by line 904. Thus, as provided herein, coherent noiseproduced by a parasitic etalon can be filtered by means of discretesampling and filtering.

In yet another embodiment, as provided herein, a reduction in spuriousspectral artifacts in the output image can also be achieved through fastsource frequency modulation and real-time detector averaging. Stated inanother fashion, a reduction in noise can be achieved by rapidly tuningthe optical source 14 to generate an illumination beam 16 having arapidly varying center optical frequency or optical frequency near atarget optical frequency (optical frequency), and slowly capturing theoutput image with the image sensor 28 during the optical frequency(optical frequency) variation. With this design, for each target opticalfrequency (optical frequency), the spectral imaging device 12 can ditherthe optical frequency (optical frequency) of the illumination beamduring the capture of the respective output image.

FIG. 10A is a graph that illustrates optical frequency versus time. Asprovided herein, the control system 30 (illustrated in FIG. 1A) cancontrol the tunable optical source 14 (illustrated in FIG. 1A) togenerate an illumination beam 16 (illustrated in FIG. 1A) having acenter optical frequency that changes in a stepped pattern from a firstoptical frequency to a tenth optical frequency and back to the firstoptical frequency over time. Somewhat similarly, FIG. 10B is a graphthat illustrates optical frequency versus time. In this example, thecontrol system 30 (illustrated in FIG. 1A) controls the tunable opticalsource 14 (illustrated in FIG. 1A) to generate an illumination beam 16(illustrated in FIG. 1A) having a center optical frequency that changesin a linear fashion from the first optical frequency to the tenthoptical frequency and back to the first optical frequency over time. Itshould be noted that the optical frequency can be adjusted in anotherfashion than illustrated in FIGS. 10A and 10B.

In these examples, the first through tenth optical frequencies are eachwithin the desired tuning range of the spectral imaging device 12(illustrated in FIG. 1A). Further, in these simplified examples, (i) attime one the illumination beam 16 has a center optical frequency of one;(ii) at time two the illumination beam 16 has a center optical frequencyof two; (iii) at time three the illumination beam 16 has a centeroptical frequency of three; (iv) at time four the illumination beam 16has a center optical frequency of four; (v) at time five theillumination beam 16 has a center optical frequency of five; (vi) attime six the illumination beam 16 has a center optical frequency of six;(vii) at time seven the illumination beam 16 has a center opticalfrequency of seven; (viii) at time eight the illumination beam 16 has acenter optical frequency of eight; (ix) at time nine the illuminationbeam 16 has a center optical frequency of nine; (x) at time ten theillumination beam 16 has a center optical frequency of ten; (xi) at timeeleven the illumination beam 16 has a center optical frequency of nine;(xii) at time twelve the illumination beam 16 has a center opticalfrequency of eight; (xiii) at time thirteen the illumination beam 16 hasa center optical frequency of seven; (xiv) at time fourteen theillumination beam 16 has a center optical frequency of six; (xv) at timefifteen the illumination beam 16 has a center optical frequency of five;(xvi) at time sixteen the illumination beam 16 has a center opticalfrequency of four; (xvii) at time seventeen the illumination beam 16 hasa center optical frequency of three; (xviii) at time eighteen theillumination beam 16 has a center optical frequency of two; and (xiv) attime nineteen the illumination beam 16 has a center optical frequency ofone.

It should be noted that one or more of the optical frequencies can be atarget optical frequency 1053. In this non-exclusive example, opticalfrequency five is the target optical frequency 1053.

FIG. 10C illustrates an output image 1013 that is captured while theillumination beam 16 (illustrated in FIG. 1A) is cycled from the firstthrough tenth optical frequency (first cycle) and back from the tenthoptical frequency to the first optical frequency (second cycle). In thissimplified example, with reference to FIGS. 10A-10C, the spectralimaging device 12 (illustrated in FIG. 1A) is controlled to capture theoutput image 1013 for the target optical frequency 1053 of opticalfrequency five while the center optical frequency of the illuminationbeam 16 is varied (dithered) cycled twice between one and ten opticalfrequencies. Alternatively, the tunable optical source 14 can becontrolled to dither the optical frequency only one cycle or more thantwo cycles during the capturing of the output image 1013. Asnon-exclusive examples, the number of cycles can be approximately 1, 2,3, 4, 5, 10, 20, 40, 50, 100, or more cycles (but the desired number ofcycles is more than 10) during a capture time of the image by the imagesensor.

In certain embodiments, the control system 30 (illustrated in FIG. 1A)modulates the tunable light source 14 (illustrated in FIG. 1A) togenerate a set of discrete modulation optical frequencies near a targetoptical frequency to produce a maximum optical frequency modulation,Δv_(modulation), about the target optical frequency set point whichsatisfies the following prescription: Δv_(modulation)=±ηΔv/2, where η isa constant having a value of greater than or equal to 0.1 and less thanor equal to 100, and Δv is the desired spectral resolution. Further, inthis embodiment, the image sensor 28 (illustrated in FIG. 1A) capturesthe output image during a capture time that is greater than thefrequency modulation.

As a non-exclusive examples, the amount of dithering about the targetoptical frequency of the modulation optical frequencies during thecapture time can be approximately plus or minus 0.1, 0.25, 0.33, 0.5, 1,2, 3, 4, 5, 6, 7, 10, or more wavenumbers.

In summary, the control system can modulate the tunable light source togenerate a set of discrete modulation optical frequencies about andthrough a target optical frequency with an optical frequency modulationrate, and the image sensor can be controlled to capture the targetoutput image during a capture time that is longer than the inverse ofthe optical frequency modulation rate.

FIG. 11 includes (i) an upper graph 1110 having a schematic illustrationof a narrow optical frequency distribution (line with narrower than theinterference that we are trying to ignore) of a typical laser outputbeam; (ii) a middle graph 1112 having a schematic illustration of a verybroad optical frequency distribution of an output beam (an ideallybroadened laser line to be approximate a top hat); and (iii) a lowergraph 1114 having a schematic illustration of a plurality of narrowoptical frequency pulses of energy generated in a relatively shortperiod of time (a laser line whose center value is shifted over time intime to produce a desired time-averaged optical frequency distributionwhich fills the spectral band Δv). Thus, provided herein, the output ofthe laser source can be extrinsically broadened using dynamic opticalfrequency modulation of laser line (dithering the laser) to produce thedesired, time-averaged optical frequency distribution 1116 (dashedline).

Thus, as provided herein, the modulation of the parasitic etalons can befiltered by fast optical frequency modulation of the laser source andreal-time detector averaging. This has an effect of averaging out theparasitic etalons and improving the resulting image quality and spectralfidelity. This embodiment has a distinct advantage in live videodiscrete frequency imaging with coherent illumination since it isbecomes unnecessary to acquire multiple frames at different opticalfrequencies and the implementation of digital signal processing toremove the noise. In many instances, the features of interest arebroader than a narrow line width. Additionally, the parasitic etalonscan be finer than the features of interest. Thus, it can be advantageousto average the unwanted spectral noise that manifests itself as a fringepattern in the spatial domain by using a broad optical frequency (e.g.modulate optical frequency over time). This can be done with either a CWlaser or a pulsed laser.

FIG. 12A is an image 1210 captured without noise reduction methodsprovided herein. It should be noted that this image 1210 includes aplurality of fringes that adversely influence the quality of the image1210.

FIG. 12B is a captured image 1212 using the spectral image device 12provided herein. The image 1212 of FIG. 12B is less influenced by thefringes.

It is understood that although a number of different embodiments of anspectral imaging device 12 have been illustrated and described herein,one or more features of any one embodiment can be combined with one ormore features of one or more of the other embodiments, provided thatsuch combination satisfies the intent of the present invention.

While the particular spectral imaging device 12 as herein shown anddisclosed in detail is fully capable of obtaining the objects andproviding the advantages herein before stated, it is to be understoodthat it is merely illustrative of some of the presently preferredembodiments of the invention and that no limitations are intended to thedetails of construction or design herein shown other than as describedin the appended claims.

1-20. (canceled)
 21. A spectral imaging device for generating one ormore, two-dimensional, images of a sample including a first image, thespectral imaging device comprising: an image sensor that includes atwo-dimensional array of sensors that capture information used toconstruct the two-dimensional spectral images; a tunable light sourcethat generates an illumination beam that follows a beam path from thetunable light source to the sample and from the sample to the imagesensor; an optical assembly including a plurality of refractive elementsthat are positioned along the beam path between the tunable light sourceand the image sensor; and a control system that (i) controls the imagesensor during a first capture time to capture first information forconstructing the first image, and (ii) controls the tunable light sourceso that the illumination beam includes a first beam set in which acenter optical frequency of the illumination beam is modulated about andthrough a first target optical frequency during the first capture time.22. The spectral imaging device of claim 21 wherein the spectral imagingdevice has a desired spectral resolution, and wherein the illuminationbeam has a spectral width that is equal to or less than the desiredspectral resolution.
 23. The spectral imaging device of claim 22 whereinthe control system controls the tunable light source so that theillumination beam includes the first beam set in which the centeroptical frequency of the illumination beam is modulated about andthrough the first target optical frequency to produce an opticalfrequency modulation, Δv_(modulation), which satisfies the followingprescription: Δv_(modulation)=±ηΔv/2, where η is a constant having avalue of greater than or equal to 0.1 and less than or equal to 100, andΔv is the desired optical frequency spectral resolution.
 24. Thespectral imaging device of claim 21 wherein the control system controlsthe tunable light source so that the center optical frequency of theillumination beam is modulated about and through the first targetoptical frequency at an optical frequency modulation rate during thefirst capture time; and wherein the first capture time is longer than aninverse of an optical frequency modulation rate.
 25. The spectralimaging device of claim 21 wherein the control system controls thetunable light source to dither about the first target optical frequencyat least two cycles during the first capture time.
 26. The spectralimaging device of claim 21 wherein an amount of dithering about thefirst target optical frequency during the first capture time is plus orminus between one-half and ten wavenumbers.
 27. The spectral imagingdevice of claim 21 wherein adjacent optical frequencies of the firstbeam set are spaced apart an optical frequency step, the opticalfrequency step being sized such that it does not produce aliasing ofFourier components of the optical frequency dependent transmittancefunction of parasitic etalons contained along the beam path into ameasurement passband, the measurement passband being equal to thereciprocal of two times a desired spectral resolution of the spectralimaging device.
 28. The spectral imaging device of claim 21 wherein thecontrol system (i) controls the image sensor during a second capturetime to capture second information for constructing a second image, and(ii) controls the tunable light source so that the illumination beamincludes a second beam set in which the center optical frequency of theillumination beam is modulated about and through a second target opticalfrequency during the second capture time; wherein the second targetoptical frequency is different than the first target optical frequency.29. The spectral imaging device of claim 21 wherein the control system(i) controls the image sensor during a plurality of capture times tocapture information for constructing each of the plurality oftwo-dimensional images, and (ii) controls the tunable light source sothat the illumination beam includes a separate beam set in which thecenter optical frequency of the illumination beam is modulated about andthrough a separate target optical frequency during the each of theplurality of capture times.
 30. The spectral imaging device of claim 29wherein the image sensor captures information for constructing at leastfive two-dimensional images.
 31. A spectral imaging device forgenerating one or more, two-dimensional, images of a sample including afirst image, the spectral imaging device comprising: an image sensorthat includes a two-dimensional array of sensors that captureinformation used to construct the two-dimensional spectral images; atunable light source that generates an illumination beam that follows abeam path from the tunable light source to the sample and from thesample to the image sensor; an optical assembly including a plurality ofrefractive elements that are positioned along the beam path between thetunable light source and the image sensor; and a control system that (i)controls the image sensor to capture a first preliminary image whilecontrolling the tunable light source so that the illumination beam has afirst center optical frequency; (ii) controls the image sensor tocapture a second preliminary image while controlling the tunable lightsource so that the illumination beam has a second center opticalfrequency that is different than the first center optical frequency; and(iii) uses the first preliminary image and the second preliminary imageto generate a first output image.
 32. The spectral imaging device ofclaim 31 wherein the first preliminary image and the second preliminaryimage are passed through a low pass filter to generate the first outputimage.
 33. The spectral imaging device of claim 32 wherein the controlsystem applies the low-pass filter utilizing at least one of a runningaverage and a Gaussian filter.
 34. The spectral imaging device of claim32 wherein the control system applies the low-pass filter utilizing atleast one of a running average and a Gaussian filter, and additionallysub-samples the data.
 35. The spectral imaging device of claim 31wherein the tunable light source emits a temporally coherentillumination beam and a desired tuning range is the mid-infrared range;and wherein the plurality of refractive elements are operable in themid-infrared range.
 36. The spectral imaging device of claim 31 whereinthe tunable light source emits a temporally coherent illumination beamand a desired tuning range is the mid-infrared range; and wherein theplurality of refractive elements are operable in the mid-infrared range.37. The spectral imaging device of claim 31 wherein the control systemthat (i) controls the image sensor to capture a third preliminary imagewhile controlling the tunable light source so that the illumination beamhas a third center optical frequency that is different than the firstcenter optical frequency and the second center optical frequency; (ii)controls the image sensor to capture a fourth preliminary image whilecontrolling the tunable light source so that the illumination beam has afourth center optical frequency that is different than the first centeroptical frequency, the second center optical frequency, and the thirdcenter optical frequency; and (iii) uses the third preliminary image andthe fourth preliminary image to generate a second output image.
 38. Thespectral imaging device of claim 37 wherein the first preliminary imageand the second preliminary image are passed through a low pass filter togenerate the first output image; and wherein the third preliminary imageand the fourth preliminary image are passed through the low pass filterto generate the second output image